Add some docs to `defstruct/1`
Add some docs to `defstruct/1`

The documentation for `defstruct/1` didn't mention that a list of atoms can be
passed as an argument instead of a keyword list. I added the missing
documentation.

kelvinst/elixir,pedrosnk/elixir,kimshrier/elixir,beedub/elixir,kimshrier/elixir,beedub/elixir,elixir-lang/elixir,ggcampinho/elixir,gfvcastro/elixir,ggcampinho/elixir,gfvcastro/elixir,pedrosnk/elixir,michalmuskala/elixir,lexmag/elixir,lexmag/elixir,joshprice/elixir,antipax/elixir,kelvinst/elixir,antipax/elixir